1. Field of the Invention
The present invention relates to a switching regulator DC-DC converter that converts DC voltage and, in particular, to a switched-mode power supply that has an adaptive on-time function and that controls output in a ripple control method.
2. Description of Related Art
A switching regulator DC-DC converter is a circuit that converts a DC input voltage and outputs a DC voltage different from the input voltage. Such a DC-DC converter has a drive switching element, a rectifying element, and a control circuit. The drive switching element applies a DC voltage supplied from a DC power source, such as a battery, to an inductor (coil) to store energy in the coil through which current flows. The rectifying element rectifies the current in the coil during energy release when the drive switching element is off. The control circuit switches on and off the drive switching element.
Conventional control methods of such a switching regulator DC-DC converter include a voltage control method in which an output voltage is fed back for modulation control of the pulse width or frequency of a drive pulse of the switching element, a current control method, which is an improved voltage control method, and a ripple control method. A speed of response to a sudden change in load is low in the voltage control method and the current control method.
In contrast, the ripple control method, which monitors an output voltage and detects a voltage below (above) a set threshold to switch on or off the switching element, is free from delay attributable to frequency characteristics of an error amplifier, thus achieving a high load response speed compared with the voltage control method and the current control method. In order to maintain the switching frequency constant while the load does not change, the ripple control method includes an adaptive on-time function that feeds forward an input voltage and an output voltage to determine the time of a timer specifying an ON time of the switching element (Japanese Patent No. 4056780).
The DC-DC converter of the ripple control method having the adaptive on-time function, however, should be provided with a terminal to apply an output voltage to the control circuit for monitoring the output voltage. Accordingly, the number of external terminals (pins) is increased in case of using an IC (semiconductor integrated circuit) for the switching control circuit, thus leading to a cost increase. Furthermore, the adaptive on-time function cannot be mounted in some cases due to limitation of the number of external terminals of the IC.